The present invention relates generally to systems for storing and dispensing gases, liquids, and/or solids. More particularly, the present invention relates to pressurized delivery systems and materials and methods for producing such systems.
Pressurized systems are widely used for dispensing a variety of consumer and industrial products including shaving cream, non-stick cooking products, hair spray, insect repellant, spray paint, and a wide variety of cleaning materials. These systems, which are usually referred to as "aerosols," commonly include three components: (1) a product to be dispensed, (2) a propellant, and (3) a pressurized container including a valve actuator (commonly a push button). The container is typically cylindrical, usually being fabricated as a sheet metal can capable of withstanding moderate pressures. In operation, a push button or other actuator opens the valve which will allow the product to be expelled from an opening or nozzle as a wet spray, fine spray, powder, foam or paste depending upon the application. The propellant, which is typically a gas under ambient conditions, is in most cases expelled from the container with the dispensed product.
The propellant and dispensed product combination may take the form of a solution, emulsion or powder within the aerosol container. In a solution, the propellant is at least partially miscible in the product. In an emulsion, the propellant forms an internal phase which vaporizes on discharge. Shaving cream is a product commonly sold with an emulsion-based dispenser system. Powder systems require a dispersion of the product in the propellant and must be specially designed to avoid clogging of the valve or nozzle during discharge.
As the above examples suggest, in most applications the propellant gas is intimately mixed with the product. In some systems, however, the propellant gas is separated from the product by a mechanical barrier such as a piston or diaphragm which prevents the propellant from passing into the product. These systems are described in European Patent Specification EP0089971 which is incorporated herein for all purposes.
A number of practical considerations limit the substances which can be used as propellant gases and/or the circumstances in which a given substance can be used as a propellant gas. Traditionally, these have included the ability to sustain pressure within acceptable limits during use, safety factors such as the flammability and toxicity of the propellant, and chemical reactivity of the propellant with the container and, mainly in the case of non-barrier dispensers, reactivity of the propellant with the product to be dispensed. For some applications, the flammability provides a particular problem. For example, many non-stick vegetable oil products will be used near an open gas flame or hot stove. Today, the environmental impact of the propellant must also be considered.
The principal advantage of liquified propellants is their ability to provide a constant pressure within the aerosol container throughout the life of the product. Chlorofluorocarbons ("CFC"s) and low molecular weight hydrocarbons such as propane, butane and isobutane are the most common liquified propellants. Because these compounds are liquid at the moderate pressures of aerosol containers, the internal pressure of the container is determined by the propellant's vapor pressure. As long as the propellant is present in liquid form within the container (typically the entire life of the product), the pressure will remain nearly constant and the product will be dispensed in the same condition. Consumers today expect this performance from aerosol containers.
Unfortunately, the liquid propellants in widespread use are either environmentally destructive or flammable and toxic. Evidence is mounting that CFCs are destroying the protective ozone layer high in the atmosphere, thus permitting dangerous levels of ultra-violet radiation to reach the earth surface. In response, the countries of the world have taken steps to curtail the use of CFCs and other commonly used propellants. Thus alternative dispensing systems will need to be developed.
One alternative to freon and hydrocarbon propellants is compressed "safe" gas propellants such as nitrous oxide, nitrogen, and carbon dioxide. Unlike the liquid propellants, these gases are nontoxic, low in cost and quite inert. Unfortunately, they can not be liquified at moderate pressures. Thus, the internal pressure of "safe" gas aerosol containers quickly decreases as the gas--and hence the product--are discharged. The rate at which the product is dispensed and the consistency (whether it be foam, aerosol spray, or liquid) will thus vary over the life of the product. Hence these products are unacceptable for most consumer uses.
As a partial solution to this pressure decay problem, liquid solvents capable of dissolving the safe gas have been included within the dispenser. In theory, the dissolved gas should come out of solution as the vapor phase gas is discharged, thus maintaining an even pressure throughout the product's life. Ethanol may, for example, be used as a solvent when cosmetic products are to be dispensed, and acetone or petroleum distillates may be used when insecticides or paints are to be dispensed. However, these applications are generally limited because the propellant is often relatively insoluble or otherwise incompatible in dispensed product.
Another attempt to employ safe gas propellants in a pressurized dispensing apparatus is described in European Patent Application 385 773 which is incorporated herein by reference for all purposes. According to that application, carbon dioxide or other gaseous propellant is stored--alone or with a liquid solvent--in a "swellable" polymeric material contained within an aerosol dispenser. The propellant is held in microvoids located between the individual molecules of the polymeric material. Thus, the polymeric material acts as a reservoir for the propellant gas, allowing greater amounts of gas to be stored within a pressurized container. As the container is discharged and the internal pressure begins to drop, the polymeric material will release the stored gas, mitigating the pressure loss, and, in theory, provide more uniform product properties over the life of the container. Unfortunately, the polymeric materials discussed in EP 385 773 swell as the container is charged, allowing only a limited amount of propellant to be sorbed before the available container space is completely occupied by the polymer/propellant composition. And on discharge, the volume of the polymer decreases resulting in less head space, less available pressure and, less efficient dispensing near the end of the product's useful life.
Accordingly, there exists a need for improved pressurized dispensing apparatus which have enhanced capacity for propellants, particularly "safe propellants as described above, but which do not suffer from the disadvantages noted above. In particular, the dispensing apparatus should employ a reservoir which does not expand to fill a major portion of the dispenser volume, and the dispenser should not exhibit substantial pressure decay during the life of the product.